1. Technical Field
The present invention relates to a DC-DC converter and a method of controlling the same. More particularly, the present invention relates to a DC-DC converter utilizing a modified Schmitt trigger circuit and a method of modulating a pulse width.
2. Description of the Related Art
DC-DC converters are used in many types of mobile electronic devices to provide a fixed output voltage. Pulse width modulation (PWM) has become a widely-used technique for controlling DC-DC converters. The DC-DC converter employing a conventional PWM method compares a reference input signal with a ramp signal outputted from an oscillator and generates a pulse signal having a duty cycle (or a duty ratio) that is proportional to the reference input signal. The pulse signal is amplified by a power switch and rectified by a filter consisting of an inductor and a capacitor to generate an output voltage that is proportional to the reference input signal. The output of the DC-DC converter is stabilized by a negative feedback circuit.
The conventional DC-DC converter that includes the oscillator for generating the ramp signal takes up a large amount of chip space in a semiconductor integrated circuit and exhibits relatively high power consumption.
Examples of DC-DC converters that generate an oscillated signal using a feedback loop without using an oscillator are disclosed in U.S. Pat. Nos. 5,481,178 and 5,770,940. FIG. 1 is a circuit diagram illustrating the conventional DC-DC converter having dual feedback loops as disclosed in U.S. Pat. No. 5,770,940.
The DC-DC converter shown in FIG. 1 is an example of a step-down switching regulator. Referring to FIG. 1, the switching regulator includes an error amplifier 106, a comparator 102, a switch 84, and an LC filter 86 consisting of an inductor 88 and a capacitor 90.
The switch 84 includes an input 92, an output line 94, and a control line 96. A diode 98 provides a current path for the inductor 88 when the switch 84 is opened. As shown in FIG. 1, the switching regulator circuit includes two feedback loops, i.e., a first feedback loop 100 including a first feedback circuit 114, and a second feedback loop 104 including a second feedback circuit 113.
The first feedback loop 100 is a quick response feedback loop that includes the comparator 102 and a driver (not shown). The comparator 102 includes built-in hysteresis, thereby providing a hysteretic window defining an upper voltage limit and a lower voltage limit.
The second feedback loop 104 is a relatively slow response feedback loop that includes the error amplifier 106. The slow response feedback loop can be used to sense a voltage at a point external to the DC-DC converter.
In the conventional DC-DC converter shown in FIG. 1, frequency of an output voltage VOUT is determined by the comparator 102 with built-in hysteresis.